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Brute Force

Learn more about Brute Force attacks

Blocking Brute Force Attacks

A common threat web developers face is a password-guessing attack known as a brute force attack. A brute-force attack is an attempt to discover a password by systematically trying every possible combination of letters, numbers, and symbols until you discover the one correct combination that works. If your web site requires user authentication, you are a good target for a brute-force attack.

An attacker can always discover a password through a brute-force attack, but the downside is that it could take years to find it. Depending on the password's length and complexity, there could be trillions of possible combinations. To speed things up a bit, a brute-force attack could start with dictionary words or slightly modified dictionary words because most people will use those rather than a completely random password. These attacks are called dictionary attacks or hybrid brute-force attacks. Brute-force attacks put user accounts at risk and flood your site with unnecessary traffic.

Hackers launch brute-force attacks using widely available tools that utilize wordlists and smart rulesets to intelligently and automatically guess user passwords. Although such attacks are easy to detect, they are not so easy to prevent. For example, many HTTP brute-force tools can relay requests through a list of open proxy servers. Since each request appears to come from a different IP address, you cannot block these attacks simply by blocking the IP address. To further complicate things, some tools try a different username and password on each attempt, so you cannot lock out a single account for failed password attempts.

Locking Accounts

The most obvious way to block brute-force attacks is to simply lock out accounts after a defined number of incorrect password attempts. Account lockouts can last a specific duration, such as one hour, or the accounts could remain locked until manually unlocked by an administrator. However, account lockout is not always the best solution, because someone could easily abuse the security measure and lock out hundreds of user accounts. In fact, some Web sites experience so many attacks that they are unable to enforce a lockout policy because they would constantly be unlocking customer accounts.

The problems with account lockouts are:

An attacker can cause a denial of service (DoS) by locking out large numbers of accounts.

Because you cannot lock out an account that does not exist, only valid account names will lock. An attacker could use this fact to harvest usernames from the site, depending on the error responses.

An attacker can cause a diversion by locking out many accounts and flooding the help desk with support calls.

An attacker can continuously lock out the same account, even seconds after an administrator unlocks it, effectively disabling the account.

Account lockout is ineffective against slow attacks that try only a few passwords every hour.

Account lockout is ineffective against attacks that try one password against a large list of usernames.

Account lockout is ineffective if the attacker is using a username/password combo list and guesses correctly on the first couple of attempts.

Powerful accounts such as administrator accounts often bypass lockout policy, but these are the most desirable accounts to attack. Some systems lock out administrator accounts only on network-based logins.

Even once you lock out an account, the attack may continue, consuming valuable human and computer resources.

Account lockout is sometimes effective, but only in controlled environments or in cases where the risk is so great that even continuous DoS attacks are preferable to account compromise. In most cases, however, account lockout is insufficient for stopping brute-force attacks. Consider, for example, an auction site on which several bidders are fighting over the same item. If the auction Web site enforced account lockouts, one bidder could simply lock the others' accounts in the last minute of the auction, preventing them from submitting any winning bids. An attacker could use the same technique to block critical financial transactions or e-mail communications.